1. Field of the Invention
The present invention relates to the field of light emitting diode (LED) power devices, and more particularly to a variable power dimming control circuit that supports the linear light dimming of a bi-directional TRI-electrode AC (TRIAC) switch and provides a holding current to stabilize the operating quality of the TRIAC switch when the LED is dimmed to a low brightness, and stops supplying the output of the holding current when the brightness of the LED is increased, so as to achieve the effects of reducing unnecessary power consumption of the circuit and minimizing the heat produced during the operation of the LED.
2. Description of the Related Art
At present, most driver circuits of an LED lamp adopt a conduction angle for inputting electric power by a TRIAC control to achieve the effect of adjusting the brightness of the LED and a current design of connecting the LED with a transistor and a sense resistor in series in order to stabilize the power and improve the quality of illumination. The sense resistor is provided for sensing a driving current passing through the LED and producing a voltage drop at both ends of the LED, and then a comparator is provided for electrically connecting or disconnecting the transistor to timely adjust the duty ratio of a pulse width modulation (PWM) signal when a high-level or a low-level switch signal is outputted to the transistor after the voltage drop is compared. Therefore, the conduction cycle of the PWM signal can be used for controlling the output of electric power that affects the magnitude of the driving current and the brightness of the LED. However, the TRIAC is a component that requires a standard operating voltage, so that if the conduction angle of the input power is too small and the overall operating voltage of the circuit has a voltage power smaller than a standard operating voltage required by the TRIAC, the TRIAC will be unable to operate normally, and the operating quality of the driver circuit will be affected adversely. To overcome these problems, the driver circuit installs an additional bleeding circuit for supplying a holding current to stabilize the TRIAC and maintaining a stable operating quality, while solving the blinking problem and expanding the dimming range.
With reference to FIGS. 1 and 2 for a circuit diagram and a waveform diagram of a conventional LED driver circuit with a holding current respectively, the driver circuit 1 comprises a dimmer 10, a bridge rectifier 11, a holding switch 12, a current resistor (RH) 13 and a filter capacitor 14. The bridge rectifier 11 receives an external power supply through the dimmer 10 and rectifies the power to produce an input voltage (Vin) and an input current (Iin). The filter capacitor 14 receives and converts the input current to produce and supply a driving current (IL) to the LED. The holding switch 10 is an N-type metal oxide semiconductor field effect transistor (N-MOSFET) having a drain coupled to the bridge rectifier 11 for receiving an input current, a source coupled to the current resistor 13, and a gate for receiving a holding voltage (VH), so that the current resistor 13 can receive the holding voltage through the holding switch 12 and output a holding current (Ihold), wherein Ihold=(VH−VGS—N-MOSFET)/RH. Therefore, the input current as shown in FIG. 2 has a constant current value Ihold in Cycle T1 (when the LED has a lower brightness or the LED is turned off) and the input current has a current value of IL+Ihold in Cycle T2 (when the LED has a higher or full brightness). In other words, regardless of the magnitude of the input power of the circuit, the holding current keeps operating, and thus causing the problems of consuming unnecessary power and lowering the power conversion efficiency of the whole circuit.
In view of the aforementioned problems, the present invention intends to improve the circuit architecture of the conventional driver circuit by using a simple detection circuit to timely adjust the holding current of the TRIAC to maintain a constant driving current value while reducing unnecessary power consumption effectively. To cope with the physical properties of the LED, the simple circuit architecture is used to provide a high voltage compensation for the input voltage, so as to achieve the effects of improving the operating quality of the whole circuit and the light emitting power of the operating quality effectively.